Mycobacterium avium subspecies paratuberculosis (MAP) is a member of the Mycobacterium avium complex MAC. Unlike other environmental MAC, MAP has the specific ability to cause chronic inflammation of the intestine of a range of histopathological types in many animals including primates. Despite its broad pathogenicity, MAP can live in animals for years without causing clinical disease. MAP is more thermotolerant than M. bovis and has been cultured from retail pasteurised milk in the UK, Czech Republic and the USA. Transmittal from livestock to humans by this route is therefore probable. There is also a high risk of transmittal from sources of environmental contamination such as rivers and surface waters used for domestic supply.
MAP can cause of Crohn's disease in humans, in particular in people who have an inherited or acquired susceptibility. Recent studies have confirmed that the inflamed intestine of most people with Crohn's disease is infected with these chronic enteric pathogens. Further studies have reported that MAP can be cultured from the blood of 50% of patients with Crohn's disease showing that, as in animals, the infection is often systemic. Furthermore a high proportion of people with Irritable Bowel Syndrome are also infected with MAP.
The organisms in humans are very slow growing and exceedingly difficult to isolate and passage in conventional culture. They are present in low abundance and adopt a Ziehl Neelsen (ZN) staining negative form which cannot be seen in tissues by ordinary light microscopy. They appear to be able to minimise immune recognition and unlike conventional spheroplasts, their ZN negative form is highly resistant to chemical and enzymatic lysis procedures essential for reliable detection by PCR.
MAP infections are extremely difficult to eradicate. ZN-negative intracellular MAP are highly resistant in vivo to standard anti-TB drugs. However, a substantial proportion of patients with Crohn's disease who can take rifabutin/clarithromycin combinations, to which MAP are more sensitive, heal, sometimes dramatically.